2/17/2014 2:19 PM PST BY TMZ STAFF

In legal docs filed by her lawyer, Miranda argues it would be unfair to put her to death -- even if convicted -- because "the criminal justice system is fallible, thereby subjecting inappropriate persons to the death penalty."

As for whether Barbour is an "inappropriate person" ... hard to make the argument since she confessed to cops. But her lawyer has an answer for that ... asking the court to throw out her confession on grounds cops violated her constitutional rights.

Barbour's lawyer says she repeatedly asked for an attorney, only to be denied.

But there's a big snag ... Barbour then confessed to knifing a man she met on Craigslist and a lot more last Thursday to a local reporter. That confession is fair game.

And one more thing -- In her lawyer's argument against the death penalty, the attorney notes the state of Pennsylvania wants her executed because this Craigslist murder was a crime of torture -- stabbing the guy 20 times. The lawyer says there's NO evidence of torture.